This keeps an "Add to Cart" button visible on screen as a customer scrolls down a product page, rather than only being available at the top.
Go to Dashboard > Products > Product Settings and find Sticky Button under Advanced Options.
Product pages with a long de script ion, FAQs, or reviews can push the add-to-cart button well below the fold. A sticky button means the customer can add the product to their cart at any point while reading, without needing to scroll back up, which reduces friction and lost sales.